POWER GENERATION AND TRANSMISSION STEAM SIZES OF ANTHRACITE COAL 35 Name Square mesh screen Average per cent of Ash Pass through, inches Pass over, inches Pea .......... ......... ys H He 15 19 25 Buckwheat No 1 Rice ..... . Barley ........................ Burning Hard Coal.—Anthracite coals require large grate areas and ample draft (page 32). In general, the cost per B.t.u. is less for coals of inferior quality: hence these coals are apt to be cheapest to use if they can be burned with efficiency. Economy in the use of fine sizes of hard coal is chiefly limited by the high ash content, which increases labor, makes it difficult to maintain a good condition of fuel bed, and increases the loss through the grate bars to the ashpit. These objections are partly offset by mixing the coal with a proportion of soft coal—run-of-mine or slack. Lignites are difficult to burn because of the incombustible nature of much of their volatile content. Extension furnaces for gradual drying of the coal are an aid. Soft Coal.—The presence of visible smoke from combustion is an indication of loss, but not necessarily of large losses. Other wastes may occur when soft coal is burned: the volatile hydrocarbons not being completely consumed. The following chart is convenient for estimating the density of smoke from chimneys, both as a check on the completeness of combustion and as evidence in case certain chimneys are attacked as nuisances by owners of property near metallurgical plants. (Use this chart at arm's length. The original is a chart 3 X 24 in., supposed to be posted about 50 ft. away.) Smoke darker than the third from the left, emitted for more than 10 per cent of the total time, has been held objectionable. FIG. 22.—Ringelmann's smoke chart. For the thorough utilization of the hydrocarbons in soft coal, there must be provided: (a) Room for the flame before it strikes the relatively cold heating surface of the boiler. The space and distance necessary vary with the amount and nature of volatile content, and other factors. As much as 12 ft. may be needed. (6) Maintenance of a steady high temperature at some point or points of the fuel bed or in the furnace.